1. Field Of The Invention
This invention relates to apparatus for perforating a well bore, and more particularly, to an apparatus for firing at least two guns with a predetermined time delay in between and preventing fluid flow between the guns.
2. Description Of The Prior Art
In well bore perforating operations, it is important for the operator to know whether or not all of the guns have been fired. Typically, a time delay device is positioned at the bottom of the string below the lowermost guns. The explosive output of one gun fires the next lower gun, and this continues with the lowermost gun triggering the time delay device. The time delay device has a relatively slow burning material in it, and after this material burns, a small explosive charge in the time delay device is detonated.
At the surface, the operator can hear and feel the guns firing, and after the guns cease firing, the operator listens for the detonation of the time delay device. If he hears it, he knows all of the guns have fired since the time delay device cannot be actuated otherwise.
A problem with this prior apparatus is that depending upon the depth and conditions of the well, the ignition of the small charge in the time delay device may not be heard. If this is the case, then the operator does not know if the last gun has fired.
In U.S. Pat. No. 4,478,294 to Sumner, assigned to the assignee of the present invention, a time delay is positioned above the lower one or two guns in the string. The firing of the last gun is thus delayed. The firing of the last gun is significantly louder, and thus more easily discerned, than the detonation of the small charge in the prior art time delay device because the charge in the perforating guns is considerably larger. Electrical sensors may also be provided to monitor the firing of the guns.
The apparatus of the present invention also uses a time delay device between upper perforating guns and at least the lowermost perforating gun. The time delay apparatus includes a housing positioned between the guns with a separate, easily replaceable, time delay device positioned in the housing.
Another problem with previous perforating tools is that once some of the guns have fired, fluid is free to enter all of the guns therebelow. If all of the guns do not fire, the fluid in the lower guns makes retrieval more difficult. Also, the entire tool must be broken down and drained in order to reuse the lower guns.
In the present invention, the time delay device prevents fluid communication between the upper guns and the gun or guns below the time delay device. This allows the time delay device and the guns therebelow to be rerun by simply replacing the portions of the perforating string thereabove. It is not necessary to break down the apparatus and drain it. In fact, the apparatus of the present invention could be run in multiple positions in perforating string with each location acting as a fluid isolation device to prevent fluid entry into the lower guns in the event of a "stop-fire" (misfire).
Prior art systems have included fluid isolation shims which perform this function, but the shims are not compatible with all of the firing devices and therefore are not usable in all situations. The present invention does not have this problem.